1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a facsimile or a copier, that forms an image using electrophotography. In particular, the present invention pertains to an image forming apparatus that includes an intermediate transfer member to which an image is first transferred from a latent image bearing member, such as a photosensitive member, and from which the image is later transferred to a recording medium.
2. Description of the Related Art
Generally, an image forming apparatus that employs electrophotography comprises: a photosensitive member, which serves as a latent image bearing unit, having a photosensitive layer on an external surface; charging means, for uniformly electrifying the external surface of the photosensitive member; exposure means, for selectively exposing the external surface that is uniformly charged by the charging means and for forming an electrostatic latent image on the external surface; developing means, for applying charged toner, as a developing agent, to the electrostatic latent image formed by the exposure means so as to provide a visible image (a toner image); and a transfer device, for transferring the toner image developed by the developing means to a recording medium, such as paper.
A well known transfer device for transferring the toner image developed on the photosensitive member to a recording medium, such as paper, is one that includes an intermediate transfer member to which the toner image formed on the photosensitive member is transferred (first transfer), and from which the toner image is transferred to a recording medium (second transfer).
Shown in FIGS. 4A and 4B is an example image of an forming apparatus that includes such an intermediate transfer member: FIG. 4A is a schematic perspective view and FIG. 4B is a cross-sectional view taken along line b--b in FIG. 4A.
In FIGS. 4A and 4B, formed on a photosensitive member 1 is a conductive layer 1a and a photosensitive layer 1b. The conductive layer 1a is grounded.
An intermediate transfer member 2 is constituted by a dielectric member (middle resistance layer) having a resistance of, for example, 10.sup.7 to 10.sup.14 .OMEGA. cm. The intermediate transfer member 2 can be fabricated by mixing conductive carbon with synthetic resin.
The intermediate transfer member 2 contacts the photosensitive member 1, at least during the image forming process, with contact point T1 serving as the first transfer point. At the first transfer point T1, a first transfer roller 3, which is located inside the intermediate transfer member 2, applies a first transfer voltage to an intermediate transfer member 2.
A second transfer roller 4, for applying a second transfer voltage, is pressed against the intermediate transfer member 2, and the point at which pressure is applied serves as a second transfer point T2. At the second transfer point T2, a backup roller 5 is located inside the intermediate transfer member 2.
In the image forming process, first, the photosensitive member 1 and the intermediate transfer member 2 are rotated, the photosensitive layer 1b of the photosensitive member 1 is uniformly electrified by charging means (not shown), and the photosensitive member is selectively exposed by exposure means (not shown) to form an electrostatic latent image. Then, a developing agent, toner, is applied to the electrostatic latent image by developing means (not shown) and a visible image (a toner image) is formed. Thereafter, at the first transfer point T1, the toner image is transferred to the intermediate transfer member 2, and at the second transfer point T2, the toner image is transferred to a recording medium, such as paper, that is fed to the second transfer point T2.
A recording medium to which the toner image has been transferred is passed through a fixing unit (not shown) and the toner image is fixed to the recording medium.
The intermediate transfer member 2 of the above described conventional image forming apparatus has a single layer structure obtained by mixing conductive particles, such as conductive carbon, with synthetic resin. Since uniform distribution of the conductive particles is not easy, the resistance value tends to vary.
Therefore, the electric field of the transfer member tends to vary, and as a result, uneven image transfers tend to occur.
In addition, since a local projection tends to be formed on the surface of the intermediate transfer member due to an agglomeration of the gel element in the resin or of the conductive particles, the contact of the photosensitive member and the intermediate transfer member, or the contact of the intermediate transfer member and the roller located on the reverse face, is locally unstable, and again, uneven image transfers tend to occur.
In order to solve the above-mentioned problems, there is an image forming apparatus as a related art for the transfer of a visible image to the intermediate transfer member at the first step, and for the transfer of the visible image to a recording medium at the second step. It is desirable for such an image forming apparatus to improve its throughput in such a way that the first transfer and the second transfer be performed at the same time accordingly.
In the arrangement wherein the intermediate transfer member is so designed that the resistance layer is integrally formed on the conductive layer, however, when the second transfer is performed during the performance of the first transfer such as a situation that the on/off state of the second transfer voltage are changed during said period, it has been found that the potential at the first transfer portion becomes unstable and spikes, and image noise is generated.